Generators with double driving machinery are known, for example, from "Allianz Report," February, 1998, page 72, and are used in power plant installations. The generator is typically arranged in what is called a single-shaft system between a gas turbine and a steam turbine, as shown in FIG. 1.2 of the cited document. The generator also includes a slip ring unit electrically connected with it, which is located on the same shaft as the generator. In such single-shaft systems, static excitation devices are installed starting with an excitation current of greater than 2000 amps, whereby the static excitation devices supply the generator rotor winding via the slip ring unit with an excitation current of up to 7000 amps. Up to an excitation current of about 2000 amps, either static excitation devices or brushless excitation machines may be used on the generator shaft.
At least on one side or on both sides of the generator shaft, the generator is connected via a transmission gear with the gas turbine and the steam turbine. Such a transmission gear in this case functions, for example, as a frequency converter to adapt 60 Hz turbines (speed: 3600 rpm) to 50 Hz generators (speed: 3000 rpm), or as a converter that converts from a supersynchronous to a synchronous speed. In addition, an SSS coupling can be used between the steam turbine and the generator, whereupon a coupling of both machines will only be possible after they reach their operating speed.
It was found that in such conventional single-shaft systems with transmission gears, the performance is limited with respect to the mechanical and electrical design. The slip ring device or the slip ring shaft cannot be constructed with a desired diameter size, since the peripheral speed of the brush face at the slip rings is limited to approximately 80 m/s. Up to 80 m/s, a good contact with the brushes for transmitting the excitation current can be ensured. For a 60 Hz generator with 2-pole design, this limit of 80 m/s is reached with a slip ring diameter of 400 mm. A slip ring diameter of 400 mm permits a slip ring shaft diameter of approximately 290 mm. But the shaft diameter of 290 mm limits the maximum mechanical power transfer to approximately 100 MW turbine power if the shaft is manufactured from a steel suitable for generator rotors. This power transmission limit takes into account both the permanent moments to be transmitted by the shaft, and interference forces, such as missynchronizations and short-circuit cutoff.
Single-shaft systems of the known type require a special mechanical design of the shaft bearing which directly adjoins the slip ring unit. The entire torque of the steam turbine is transmitted via this shaft bearing to the generator, whereby the slip ring unit represents only a comparatively small weight load, and the shaft bearing therefore requires a correspondingly complex design.
In addition, a conventional slip ring shaft has, on one side, a coupling flange that has been worked in one piece from this shaft, and, on the other side, a welded-on or shrunk-on coupling that is attached to the slip ring shaft after the slip rings have been installed. The slip rings cannot be installed on a slip ring shaft over a coupling flange.